The present invention relates to ceramic articles and, more particularly, to the manufacture of porous refractory ceramic articles by processes including a procedure for eliminating, prior to the firing of the articles, compounds or components which, while useful and economical in preparing the green or unfired article, detract from the thermal shock resistance and durability of the final fired ceramic article in its intended use.
In the formation of ceramic articles, a wide variety of processes and compositions are available for selection depending upon intended end use of the article and particular considerations regarding ease of processing, economics and the like.
Ceramics are well known for their compositional and dimensional stability under high temperature conditions.
Of late, interest has increased in the development of porous ceramic compositions either for use as filtering elements in aggressive or stringent environments, or for other uses where the stability and durability of ceramics are desired but where a low mass or density also is required or desirable. Processes are known for producing porous ceramic articles, such as by formulation of a precursor composition containing constituents which are burned out during firing to leave behind pores or voids; the use of pre-prepared porous organic foams (e.g., polyurethane) as molds or carrier for liquid ceramic compositions, whereby the foam is burned out during firing to leave behind a porous ceramic in the shape and form of the carrier; and foamable castable ceramic compositions which can be poured into a mold, foamed by virtue of external agents (e.g., air) or in situ gas-generating reactions, set in the porous configuration in the mold, and fired.
A general characteristic of such processes is their expense, generally attributable to the need for numerous processing steps. To minimize this expense, it is generally desirable to employ the lowest cost ingredients possible, and to this end it frequently will be desirable to utilize in the ceramic composition compounds in readily-available (hence, inexpensive) forms, such as alkali metal salts thereof. In other situations, compounds such as these are required in order to bring about particular effects. A particular example of this is found in castable compositions which set as a result of formation of a silicate or aluminosilicate hydrogel (see, e.g., U.S. Pat. Nos. 4,357,165 and 4,432,798, incorporated herein by reference), wherein there are deliberately added to the composition water-soluble forms (e.g., alkali metal salts) of silica and alumina, such as sodium aluminate and sodium silicate.
Although economically expedient or functionally expedient, difficulty arises because compounds of this type at the same time lead to glassy phases in the microstructure of the fired ceramic article which substantially reduce the thermal resistance and durability of the article for which it was supposed to be designed. As a consequence, if use of such compounds is desired, it is necessary to devise a way to eliminate or alter these compounds prior to firing to arrive at a suitably refractory ceramic article.
As will be discussed in further detail hereinafter, the present invention provides a process for removing and/or altering alkali metals or compounds of alkali metals from a green, unfired porous ceramic shape, such that upon firing a refractory porous shape is produced. The invention has particular applicability to shapes prepared from silicate-containing compositions, for example, utilizing silicate-containing binder compositions, but also has applicability to any ceramic composition containing leachable and/or ionicallyexchangeable alkali metals or alkali metal compounds or other compounds which otherwise would lead to formation of glassy phases in fired porous ceramic articles. As will be further described, the process of the invention involves the water leaching of an alkali-containing porous green ceramic, followed by treatment to effect exchange of ammonium ion for sodium ion utilizing dilute solutions preferably of ammonium chloride, but talso of ammonium acetate or bicarbonate or carbonate or hydroxide or sulfate, or mixture of such ammonium salts.
The prior art contains reference to a number of techniques for dealkatization of silicate compositions, although not strictly in the context of preparing refractory ceramic articles. For example, in a paper entitled "Durable Glass By Reconstitution Of Hydratable Sodium Silicate Glasses," published by the American Chemical Society in 1982, Bartholomew, et al. discuss a procedure utilizing either sodium or ammonium nitrate solutions to reduce the alkali content of 12-21% Na.sub.2 O-containing hydrated sodium silicate glasses by 50% over a several hour treatment period, as well as a more efficient process utilizing a 0.6 molar solution of ammonium nitrate at 80.degree. C. to reduce the alkali content of a 2 mm thick hydrated sodium silicate glass, containing 21% sodium oxide, to about 100 ppm sodium in a 24-hour period.
In U.S. Pat. No. 4,340,642 to Netting, there is disclosed a process wherein aluminum sulfate is used to enrich the surface layer of sodium silicate-derived spherical particles.
In U.S. Pat. No. 3,653,864 to Rothermal, alkali metal oxide-containing glasses are dealklzed by first hydrating the alkali to a hydroxide, preferably by autoclaving, and then by contacting the glass surface with a non-acidic, non-aqueous solvent having a high dielectric constant, such as formamide or dimethyl formamide.